From U.S. Pat. No. 6,253,837 B1 a bypass valve of the type described is already known, in which the valve spring that pushes the closure element to its closing position is supported on a thermo-sensitive control drive, which during normal operation, i.e. when the hydraulic fluid is within its normal temperature range, acts upon the valve spring so that the latter presses the closure element against an associated valve seat, thereby closing the bypass. In this operating mode the hydraulic fluid coming from the hydraulic aggregate passes through the cooler and back again to the hydraulic aggregate. If the temperature of the hydraulic fluid falls below the minimum desired operating temperature, the thermo-sensitive control drive relaxes the force on the valve spring sufficiently for the closure element, under the action of the restoring spring, to be pushed to an open position and the bypass is opened, so that at least some of the hydraulic fluid coming from the hydraulic aggregate bypasses the cooler. In one embodiment the inlet line to the cooler is at the same time closed so that the cooler is cut off. In such a case all the hydraulic fluid bypasses the cooler and flows back, uncooled, into the hydraulic aggregate. The thermo-sensitive control drive is of the piston-cylinder type, the cylinder being filled with a thermo-sensitive material, for example wax, which expands or contracts more or less as a function of the temperature. The closure element is made as a ring arranged to move along a shaft of the control drive, so that if pressure peaks occur in the hydraulic fluid it can be lifted clear of the valve seat even though the thermo-sensitive control drive is in the bypass-closing position.
The design and structure of the known bypass valve are relative elaborate and it is therefore comparatively expensive to manufacture. Moreover, owing to the use of a separate control drive and owing to the displaceable mounting of the closure element, it is prone to malfunction.